Since Ni-base alloys are excellent in corrosion resistance and mechanical properties, they have been used for the material of various members. In particular, the Ni-base alloys has been used for atomic reactors, since when it is exposed to high temperature water, it has excellent corrosion resistance. For example, as a heat exchanger tube for a steam generator in the pressurized water reactor (PWR), alloy 690 (trade name), i.e., 60% Ni—30% Cr—10% Fe, is used.
These members are used in high temperature water of about 300° C., which is the environment of the reactor water, for several years for shorter life and for tens years for longer life. Although the Ni-base alloy is excellent in corrosion resistance and has a small corrosion rate, some amount of Ni may be released from the alloy as Ni ions during a long period of time.
The released Ni is carried to the core of the reactor in the circulating process of the reactor water and is irradiated with neutrons in the vicinity of nuclear fuel. When Ni is subjected to the neutron irradiation, it is converted to Co by a nuclear reaction. Since Co has a very long half-life, it continues to emit radiation for a long period of time. Therefore, when the amount of released Ni is large, the dosage of radiation to workers, who carry out periodical inspections and the like, increases.
It is very important to reduce the dosage of radiation when using the light water reactor for a long period of time. Therefore, some measures to prevent the Ni release from the Ni-base alloy, such as an improvement of corrosion resistance of the alloy and controlling the water quality in the atomic reactor have been adopted.
The Japanese laid-open patent publication Sho.64-55366 discloses a method of improving general corrosion resistance by annealing a heat exchanger tube of Ni-base alloy in an atmosphere of a vacuum degree of 10−2 to 10−4 torr, at a temperature range of 400 to 750° C., in order to form an oxide film mainly consisting of chromium oxide. Further, the Japanese laid-open patent publication Hei.1-159362 discloses a method of improving intergranular stress corrosion cracking resistance. In the method, oxygen of 10−2 to 10−4 volume % is introduced into an inactive gas for heat treatment, and the alloy is heat-treated at a temperature range of 400 to 750° C. to produce an oxide film consisting mainly of chromium oxide (Cr2O3).
The Japanese laid-open patent publications Hei.2-47249 and Hei.2-80552 disclose methods of suppressing the dissolution of Ni and Co in the stainless steel for a super-heater tube by heating it in an inert gas containing a specified amount of oxygen, in order to form a chromium oxide film.
The Japanese laid-open patent publications Hei.3-153858 discloses a dissolution resistant stainless steel in high temperature water. The stainless steel is provided with an oxide layer, which contains more amounts of Cr-containing oxide than oxide that does not contain Cr, on its surface.
All of these methods reduce the amount of released metals by forming an oxide film consisting mainly of Cr2O3 by heat treatment. However, the Cr2O3 films obtained by the methods lose the release preventing effect by damaging the film over a long period of time. The reasons are considered to be insufficient film thickness, an inadequate film structure and a small amount of Cr content in the film.
The Japanese laid-open patent publications Hei.4-350180 discloses a method of reducing the discharge of gas components from the inside surface of the stainless steel tube for extra-high-purity gas. In this method, electro-polished stainless steel tubes on their inside surface, the so-called EP tubes, are sequentially connected to each other and subjected to a solution heat treatment, while continuously supplying hydrogen gas into the tube, in order to form a passive film consisting mainly of Cr2O3. According to this method, a uniform passive film can be easily formed. However, since a pretreatment, such as the electro-polishing for high cleanliness of the tube requires large manpower, the production costs increase.